The present invention relates to a semiconductor device and a method of fabricating the same.
Recently, the use of silicon germanium (SiGe) obtained by adding germanium (Ge) to silicon (Si) as a gate electrode material of a P-channel MOS transistor (to be referred to as a PMOSFET hereinafter) is proposed. This makes it possible to increase the activation ratio (the ratio of activated impurity atoms to all doped impurity atoms) of boron (B) as a P-type dopant, and thereby decrease the thickness of a depletion layer formed near the interface of the gate electrode and suppress depletion of the gate electrode.
In the surface portion of the gate electrode, a silicide for reducing the parasitic resistance is formed. Since silicon germanium (SiGe) does not well match a silicide made of cobalt (Co) or nickel (Ni), the resistance of the silicide increases. Therefore, a method is proposed by which a silicon (Si) film as a silicide reaction layer to be reacted with a silicide is formed on a silicon germanium (SiGe) film, thereby forming a stacked structure of the silicon germanium (SiGe) film and silicon (Si) film as a gate electrode.
If, however, predetermined annealing is performed after the silicon germanium (SiGe) film and silicon (Si) film are formed, germanium (Ge) in the silicon germanium (SiGe) film diffuses in the silicon (Si) film as an upper layer. If the germanium (Ge) concentration in the silicon (Si) film as an upper layer exceeds about 5 at %, the resistance of a silicide to be formed later undesirably increases. Note that at % represents an atomic composition ratio.
To prevent germanium (Ge) in the silicon germanium (SiGe) film from diffusing in the silicon (Si) film as an upper layer, therefore, the germanium (Ge) concentration in the silicon germanium (SiGe) film as a lower layer is desirably as low as possible, and the film thickness is desirably as small as possible.
Unfortunately, in a PMOSFET, to suppress depletion of the gate electrode by increasing the activation ratio of ion-implanted boron, it is necessary to increase the germanium (Ge) concentration in the silicon germanium (SiGe) film. Accordingly, the formation of a silicon germanium (SiGe) film having a low germanium (Ge) concentration causes depletion of the gate electrode, and makes it difficult to improve the drivability of the transistor.
If, therefore, a silicon germanium (SiGe) film having a high germanium (Ge) concentration and a small film thickness is formed near the interface with a gate insulating film, it is possible to prevent an increase in resistance of the silicide because the amount of germanium (Ge) which diffuses in the silicon (Si) film as an upper layer reduces. In addition, depletion of the gate electrode can be suppressed since the activation ratio of boron increases.
Unfortunately, a silicon germanium (SiGe) film having a high germanium (Ge) concentration and a small film thickness readily causes migration because the melting point of germanium (Ge) is as low as about 945° C. As a consequence, a large number of projections and recesses are formed on the film surface, and this worsens the morphology (the surface state).
By contrast, when silicon germanium (SiGe) is used as the gate electrode material in an N-channel MOS transistor (to be referred to as an NMOSFET hereinafter), the activation ratio of phosphorus (P) or arsenic (As) as an N-type dopant decreases.
Accordingly, to form a complementary MOS transistor (to be referred to as a CMOSFET hereinafter) made up of a PMOSFET and NMOSFET, it is desirable to form a stacked structure of a silicon germanium (SiGe) film and silicon (Si) film as a gate electrode of the PMOSFET, and form only a silicon (Si) film as a gate electrode of the NMOSFET.
If, however, a silicon germanium (SiGe) film having a high germanium (Ge) concentration and a small film thickness is formed, the morphology worsens, so a silicon germanium (SiGe) film having a high germanium (Ge) concentration and a large film thickness must be formed below the gate electrode of the PMOSFET. In this case, the heights of the gate electrodes of the PMOSFET and NMOSFET are largely different, and this makes the formation of these gate electrodes impossible.
A reference concerning a MOSFET which uses silicon germanium (SiGe) as the gate electrode material is as follows.
Reference 1: Japanese Patent Laid-Open No. 2002-343881